Graphene applications in chemical sensing are based on the chemical doping of graphene. In this process, molecules adsorbed on graphene serve as charge-carrier donors or acceptors, thus changing the graphene conductivity. While the previous studies have been focused on chemical sensors with electrical detection, we theoretically investigate chemical sensing based on photonic structures covered with graphene. By considering chemical doping of graphene as a small perturbation, we show that optimal photonic structures operate at low-terahertz frequencies, with the reflectance intensity as the output signal. In order to achieve an efficient chemical sensing, photonic structures should provide the electric-field enhancement within the graphene plane. As a result, the proposed structure consists of the metallic mirror and quarter-wavelength-thick dielectric spacer with graphene on the top of it. The sensitivity is maximized when the Fermi energy in the graphene not exposed to the environment is around 30 meV. By taking the resolution for the reflectance measurement of 1%, we show that the proposed sensing structure can detect graphene doping by 150 electrons or holes per square micrometer in the dynamic range of around 3000 charge carriers.

The new generation of x-ray free-electron lasers opens up unique avenues for exploring matter under exotic and extreme conditions. Extensive spatial characterization of focused, typically (sub)micron-sized, laser beams is indispensable but, nevertheless, difficult to be accomplished due to excessive radiation intensities. Methods exist allowing indirect or semidirect focus characterization from a safe distance far from the focal point. Here we present a direct method of in-focus numerical phase recovery exploiting multishot desorption imprints in poly(methyl methacrylate). Shapes of the imprints serve as input data for the newly developed code PhaRe (phase recovery), inspired by the iterative Gerchberg-Saxton algorithm. A procedure of dynamic input-output mixing guarantees that the algorithm always converges to a self-consistent paraxial Helmholtz equation solution, which is thereafter optimized for transverse spatial coherence. Very good agreement with single-shot ablation imprints in lead tungstate (${\mathrm{PbWO}}_4$) is found. The experiment is carried out at the Linac Coherent Light Source with a focused beam monochromatized at 800 eV. The results of the coherence optimization indicate that the act of monochromatization may have an effect on otherwise very good transverse coherence of free-electron laser beams.

Near-field heat engines are devices that convert the evanescent thermal field supported by a primary source into usable mechanical energy. By analyzing the thermodynamic performance of three-body near-field heat engines, we demonstrate that the power they supply can be substantially larger than that of two-body systems, showing their strong potential for energy harvesting. Theoretical limits for energy and entropy fluxes in three-body systems are discussed and compared with their corresponding two-body counterparts. Such considerations confirm that the thermodynamic availability in energy-conversion processes driven by three-body photon tunneling can exceed the thermodynamic availability in two-body systems.

By adding photonic wire structures to an organic microcavity, we create an additional confinement and a Bloch-like band structure in the dispersion of periodically structured cavities. We experimentally observe spontaneous and stimulated emission from the ground and different excited discrete modes at room temperature. By changing the spatial gain distribution via a two-beam interference, we are able to directly control the laser emission from both extended and confined modes of such organic photonic wires. Both spatial distribution and dispersion exhibit coherent emission from tunable modes, which we describe with an analytical model and numerical simulations, in agreement with our measurements.

We demonstrate ultrabroadband and ultrafast midinfrared (MIR) pump-probe spectroscopy by employing the chirped-pulse up-conversion technique with four-wave difference-frequency generation in a gas. The full spectra of the MIR probe pulses, which spread from 200 to $5000{\mathrm{cm}}^{-1}$, are recorded on a single-shot basis. We apply this MIR pump-probe spectroscopy to measure the ultrafast carrier dynamics of an intrinsic germanium crystal wafer. Ultrafast reflectivity change induced by the excitation of carriers is observed in the region from far to near infrared. The behavior of the reflectivity-change signals at each delay time is explained by the Drude model.

The generation of random numbers via quantum processes is an efficient and reliable method to obtain true indeterministic random numbers that are of vital importance to cryptographic communication and large-scale computer modeling. However, in realistic scenarios, the raw output of a quantum random-number generator is inevitably tainted by classical technical noise. The integrity of the device can be compromised if this noise is tampered with or even controlled by some malicious party. To safeguard against this, we propose and experimentally demonstrate an approach that produces side-information-independent randomness that is quantified by min-entropy conditioned on this classical noise. We present a method for maximizing the conditional min entropy of the number sequence generated from a given quantum-to-classical-noise ratio. The detected photocurrent in our experiment is shown to have a real-time random-number generation rate of $14(\mathrm{Mb}\mathrm{i}\mathrm{t}/\mathrm{s})/\mathrm{MHz}$. The spectral response of the detection system shows the potential to deliver more than $70\mathrm{Gbit}/\mathrm{s}$ of random numbers in our experimental setup.

We study the self-heating of a large stack of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ intrinsic Josephson junctions, of a configuration designed for terahertz generation. We find good qualitative agreement between direct thermoluminescent measurements of the device surface temperature and low-temperature scanning laser microscopy images. In particular, the two techniques both reveal a mode of thermal instability through the asymmetric nucleation of a small hot spot near a corner or edge of the sample. This behavior conforms with a theoretical stability analysis, and the radius of the hot spot is in excellent agreement with theoretical predictions, as is its growth with increasing bias current and bath temperature. Narrow hot spots may offer a possible means of enhancing the terahertz emission power from this type of device.

The optimized soliton mode locking of the erbium-doped fiber laser (EDFL) and its pulse-shortening dynamic with the graphene nanosheet is demonstrated by precisely detuning the weakly negative group-delay dispersion (GDD) and maintaining strong self-phase-modulation (SPM), to obtain the shortest pulse width of 449 fs with a spectral linewidth of 6.02 nm. The pulse evolution with the mode-locking mechanism changing from the self-amplitude-modulation of the saturable absorber, to the soliton compression caused by the GDD and SPM is experimentally and numerically investigated in detail. Under high pumping powers, the enlarged up-chirp inside the EDFL cavity can induce a significant Kelly-sideband shift of up to 0.5 nm. The passively-mode-locked EDFL pulse width is controllable by detuning the GDD and SPM parameters, so that the pulse width can be compressed from 642 to 449 fs while reducing the negative GDD from $-0.354$ to $-0.154{\mathrm{ps}}^2$. The compression ratio can be also improved by strengthening the SPM at this stage.

The intrinsic sensitivity of whispering-gallery-mode resonators aimed at measuring refractive index can be extremely high, although their practical performance is compromised by temperature fluctuations that masquerade as refractive-index changes. We present a triple-mode approach that delivers simultaneous and independent sensing of temperature and refractive-index changes in the same resonator. The frequency difference between two orthogonally polarized modes is used to sense temperature which is then actively stabilized to $\sim 1\mu \mathrm{K}$ over 15 minutes. We then detect a frequency difference between two modes of different wavelengths to obtain a refractive-index measurement that is free of temperature fluctuations. This triple-mode technique delivers a state-of-the-art detection limit of $8\times 10^{-9}$ refractive-index units, despite the resonator size being 100 times larger than that typically used for sensitive refractometric sensing.

We report on tuning the THz emission of a ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ (BSCCO) intrinsic Josephson-junction stack by a focused laser beam which is scanned across the stack. The emission power $P_e$ increases by up to 75% upon laser irradiation for a bath temperature near 22 K. The laser-induced changes in the voltage $V_{\mathrm{dc}}$ across the stack and in the emission power are measured simultaneously. The maximum of the laser-induced changes in emission power $\mathrm{\Delta }{P}_e$ is achieved by irradiating the stack on the location where the local temperature is about the critical temperature $T_c$. However, $\mathrm{\Delta }{P}_e$ is found to be proportional to the laser-induced global voltage change $\mathrm{\Delta }{V}_{\mathrm{dc}}$, irrespective of the laser position. This unexpected global response is likely to be related to a change in the average stack temperature and is consistent with the change in $P_e$ when increasing the bath temperature by about 0.2 K. This tuning method can be employed in the application of BSCCO THz sources.

A conceptual paradigm for multiparametric optical biosensing in a lab-on-a-chip, using the cascaded transmission of light through a photonic crystal that contains microfluidic channels and is embedded in a glass slide, is demonstrated. Accurate detection of multiple disease-identifying biomarkers is facilitated by the interaction of surface and photonic-band-gap waveguide modes. Through finite-difference time-domain simulations, levels of light transmission through the device are shown to be simultaneously responsive to analyte bindings and layer thicknesses at different locations along a single optical transmission path through the photonic crystal. Our multiparametric biosensing mechanism supersedes traditional single-resonance-shift-based biosensing and provides a more detailed spectral fingerprint of various diseases or various stages of a given disease. Moreover, the spectral line shape due to the engineered optical modes can logically discriminate between different concentrations of several analytes flowing through the microfluidic channels. The simultaneous detection and differentiation of (combinations of) distinct analytes using a single measurement on one device offers a paradigm for optical biosensing.

We investigate the use of guided modes bound to defects in photonic crystals for achieving double resonances. Photoluminescence enhancement by more than 3 orders of magnitude is observed when the excitation and emission wavelengths are simultaneously in resonance with the localized guided mode and cavity mode, respectively. We find that the localized guided modes are relatively insensitive to the size of the defect for one of the polarizations, allowing for flexible control over the wavelength combinations. This double-resonance technique is expected to enable the enhancement of photoluminescence and nonlinear wavelength-conversion efficiencies in a wide variety of systems.

We report the creation of a low-loss broadband optical antenna giving highly directed output from a coherent single spin in the solid state. The device, a crystalline solid-state realization of a dielectric antenna, is engineered for individual nitrogen-vacancy electronic spins in diamond. We demonstrate a directionality close to 10. The photonic structure preserves the high spin coherence of single-crystal diamond ($T_2\gtrsim 100\mu \mathrm{s}$). The single-photon count rate approaches a megahertz facilitating efficient spin readout. We thus demonstrate a key enabling technology for quantum applications such as high-sensitivity magnetometry and long-distance spin entanglement.

Plasmon resonances are widely used to enhance the magneto-optical responses of ferromagnetic materials. Yet, the possibility of exploiting plasmonics to boost the magneto-optic signals of diamagnetic systems is far less common. Here, we develop a comprehensive effective-medium model that anticipates with high accuracy both the ferromagnetic and diamagnetic magneto-optical responses of magnetic colloids. Experiments validate our model, for which the effective-medium Verdet constant is strongly increased by localized plasmon resonances. Our work underlines the potential of plasmon-enhanced diamagnetic optical responses for high-magnetic-field applications and plasmon-based detection.

A semiclassical simulation approach is presented for studying quantum noise in large-scale photonic circuits incorporating an ideal Kerr nonlinearity. A circuit solver is used to generate matrices defining a set of stochastic differential equations, in which the resonator field variables represent random samplings of the Wigner quasiprobability distributions. Although the semiclassical approach involves making a large-photon-number approximation, tests on one- and two-resonator circuits indicate satisfactory agreement between the semiclassical and full-quantum simulation results in the parameter regime of interest. The semiclassical model is used to simulate random errors in a large-scale circuit that contains 88 resonators and hundreds of components in total and functions as a four-bit ripple counter. The error rate as a function of on-state photon number is examined, and it is observed that the quantum fluctuation amplitudes do not increase as signals propagate through the circuit, an important property for scalability.